Although heart cell cultures have been used extensively for screening cardioactive drugs, the cells propagated in culture have undergone physiological changes such that they no longer respond the same as in situ. The recent development of organ culture techniques applied to mouse embryo hearts provides a means for maintaining active functional hearts for periods of two to four weeks. The organ culture technique offers a better model for drug screening, however, the cultured hearts are opaque and the details of contraction cannot be directly observed with the light microscope. The development of the ultrasonic microscope permits the real time examination of the dynamic aspects of isolated mouse embryo hearts in detail not permitted by any other method. The ultrasonic microscope operates at either 100 or 250 MHz. This high magnification high resolution system permits the examination of the structural detail of the living heart. The hearts, dissected from 19 to 21 day fetal mice, are placed in culture chambers which maintain appropriate physiological conditions within a constant flow chamber. The hearts beat spontaneously and rhythmically in a commercially available media to which serum and insulin have been added. A life of about 20 days for these isolated hearts gives ample time to study cardioactive drugs. The culture chambers can be transferred to the stage of the ultrasonic microscope for detailed examination of the contractile events. The drugs and other agents to be studied are added to the medium which flows through the culture chamber. Changes in the performance of the heart can be studied by recording electrical and mechanical activities while at the same time observing modification to the myocardial activity.